The technology of Multimedia Broadcast Multicast Service (MBMS) is intended to provide a user in radio cellular systems with a multimedia broadcast and multicast service. The MBMS in a Long Term Evolution (LTE) system is also referred to as an evolved MBMS (eMBMS). The eMBMS in an existing LTE Release 9 (LTE R9) system is performed in MBMS/Unicast-mixed cells, that is, a unicast service can also be transmitted in a cell where an MBMS is transmitted to perform coordinated transmission of the unicast service and the MBMS by transmitting the MBMS at some instances of time and the unicast service at other instances of time. Furthermore, the MBMS is transmitted in the LTE R9 system with Multicast Broadcast Single Frequency Network (MBSFN) mode, that is, the same content of information is transmitted concurrently in multiple cells at the same frequency at the same time.
A logical channel is such an interface to a high layer protocol that is defined at a Media Access Control (MAC) layer which provides the logical channel with a data transmission service. The type of transmitted data service on different logical channel is distinguished by the type of the logical channel. A transport channel is an interface provided at a physical layer for a high layer (e.g., the MAC layer) to transmit data, and the type of the transport channel is defined dependent upon a transmission characteristic and a transmission scheme of the data at the physical layer.
In the LTE system, existing logical channels involved in transmission of an MBMS generally include a Broadcast Control Channel (BCCH), a Multicast Control Channel (MCCH) and a Multicast Traffic Channel (MTCH). Transport channels generally include a Multicast Channel (MCH), a Broadcast Channel (BCH) and a Downlink Shared Channel (DL-SCH), all of which are downlink channels.
In the LTE R9 system, logical channels and transport channels have a mapping relationship as illustrated in FIG. 1, where the BCCH is mapped onto the BCH or the DL-SCH, and in the MBMS/Unicast-mixed cells, the MTCH and the MCCH are mapped onto the MCH. These channels primarily function as follows.
(1) BCCH
System information is transmitted over the broadcast control channel from a network to a User Equipment (UE). For the MBMS, configuration information of the MCCH, e.g., the repetition period of the MCCH, an offset of a starting sub-frame of the MCCH, an amount of sub-frames occupied by the MCCH, and a configuration of Radio Link Control Unacknowledged Mode (RLC UM), is carried over the BCCH so that the UE can locate the resource of the MCCH, but no specific MBMS traffic information is carried over the BCCH.
(2) MCCH
The multicast control channel is a point-to-multipoint downlink channel over which MBMS-related control information is transmitted to the UE from the network, and one MCCH may correspond to one or more MTCHs, that is, control information of a plurality of MTCHs may be carried over the MCCH.
Information carried over the MCCH includes configuration information of an MBMS logical channel (e.g., the MTCH, and an MSCH), information of an MBMS Radio Bearer (RB) (e.g., the RB index information of RBs provided in a modification period of the MCCH), information of an MBMS Session Start, and information of an MBMS Session Stop.
(3) MTCH
The multicast traffic channel is a point-to-multipoint downlink channel over which specific MBMS traffic data is transmitted to the UE from the network.
The MTCH and the MCCH may be mapped onto the same MCH, that is, the MTCH and the MCCH are multiplexed at the MAC layer. The MTCH and the MCCH may be multiplexed at the MAC layer through Time Division Multiplexing (TDM), or TDM-based internal multiplexing in a Transport Block (TB).
Particularly, transmission of data over a transport channel through TDM refers to time-division transmission of data of logical channels multiplexed over the same transport channel, that is, data of only one of the logical channels multiplexed over the transport channel can be transmitted in one sub-frame allocated to the transport channel. With multiplexing through TDM, data information over the MTCH and the MCCH is transmitted respectively over different sub-frames corresponding to the MCH, as illustrated in FIG. 2.
As illustrated in FIG. 3, if the entire resource of a sub-frame can not be occupied by data of a logical channel, the remaining part of the sub-frame shall be filled with padding bits. Therefore, transmission of data over a transport channel through TDM-based internal multiplexing in a TB means that firstly data of a specific logical channel (or a set of logical channels), e.g., data of the MCCH, or a combination of data of the MCCH and another logical channel (e.g., the MBMS Scheduling Channel (MSCH) over which scheduling information of an MBMS is transmitted), is transmitted through TDM, and when all of the data over the specific logical channel or the set of logical channels in a specific sub-frame has been transmitted but the resource available from the TB of the sub-frame has not been exhausted, data of another logical channel (or a set of logical channels) is allowed to be transmitted in the same TB.
In the existing LTE system, one sub-frame corresponds to one TB for each transport channel, and only one Modulation and Coding Scheme (MCS) is adopted for the TB. Data of only one MCH transport channel may be present in an MBSFN sub-frame occupied for MBMS transmission. Since MBMS-related control information is carried over the MCCH, a different modulation and coding scheme from that for transmission of data of another logical channel (e.g., the MTCH) is typically adopted to improve the reliability of reception at the UE so that the UE can receive the MBMS-related control information reliably. However, when data of logical channels, such as the MCCH and the MTCH, is multiplexed over the same MCH, if a modulation and coding scheme preset for the MCCH is adopted for all of the sub-frames of the MCH, the spectrum efficiency will be low; or if a modulation and coding scheme preset for another logical channel (or a set of logical channels), e.g., the MTCH, is adopted for all of the sub-frames of the MCH, the reliability of the MCCH will fail to be ensured.
In summary, it is impossible in the existing LTE system to satisfy different transmission quality requirements of data over a plurality of types of MBMS-related logical channels mapped onto the same transport channel for which only one MCS can be adopted.